The disclosure of Japanese Patent Application No. 11-267031 filed on Sep. 21, 1999, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a three-dimensional cam having a profile shape that varies along a rotating axis thereof, and a production method for the cam.
2. Description of Related Art
In order to improve the performance of internal combustion engines, continuously variable valve apparatus have recently been proposed that change valve characteristics, such as amount of lift, open/close timing, open valve period, etc., of engine valves, based on the three-dimensional configurations of cams.
Internal combustion engines equipped with variable valve apparatus as described above employ, as cams for opening and closing engine valves, three-dimensional cams having cam profile shapes that continuously vary along their rotating axes. By moving a camshaft connected to such a three-dimensional cam in a direction of the rotating axis thereof by hydraulic pressure or the like, the apparatus changes the cam profile shape in contact with a valve lifter of the engine valve. In accordance with changes of contact cam profile, changes occur in the open/close timing, the open/close amount, the open/close duration, etc., of the intake or exhaust valve driven by the valve lifter.
Since the cam profile shape of a three-dimensional cam varies along the rotating axis, high-precision processing of the cam profile surface of the cam is very difficult. For example, if a cam profile surface is machined by grinding with a grindstone as described in Japanese Patent Application Laid-Open No. 10-44014, complicate process steps and an increased process time are needed in order to secure a sufficient precision.
Therefore, a three-dimensional cam is formed through integral molding by a powder metallurgy (i.e., net-shape-sintering). The net-shape-sintering allows highly efficient production of three-dimensional cams having complicated cam profile shapes while securing sufficient precision.
In general, cams used for opening and closing engine valves of internal combustion engines, not confined to three-dimensional cams, are required to have high durability against damage, such as slide abrasion, pitting and like, because these cams are rotated at high speeds while being pressed against valve lifters by valve springs of the engine valves and therefore receive high surface pressures. Particularly, three-dimensional cams used in a continuously variable valve apparatus need to have further high durability because the cams also are moved in the direction of the rotating axis during operation of the internal combustion engine.
Although cams formed by the aforementioned net-shape-sintering process have higher durability than normally employed cast cams, a further improvement in durability is desired because operation of cams in even more severe conditions is demanded in order to improve the performance of internal combustion engines.
The invention has been accomplished in view of the aforementioned circumstances. It is an object of the invention to provide a three-dimensional cam having a cam profile that varies along (i.e., in the direction of) its rotating axis, and a production method for the cam that allow a further improvement in durability while securing high productivity.
To achieve the aforementioned and/or other objects, a three-dimensional cam according to a first aspect of the invention has a cam profile that varies along a rotating axis, and is produced by net-shape-sintering. The cam profile surface has holes at a proportion of 5 to 10% relative to the total area of the cam profile surface.
Net-shape-sintering, that is, integral formation by powder metallurgy, is able to form a three-dimensional cam having a complicated cam profile with a high form precision, without necessitating a machining process, and therefore is able to secure a high productivity. Since the three-dimensional cam is produced by net-shape-sintering, the construction of the first aspect of the invention is able to improve productivity while securing sufficient precision of the three-dimensional cam.
Furthermore, according to the first aspect, the three-dimensional cam produced by net-shape-sintering has, in its cam profile surface, holes at a hole rate of 5 to 10%. The hole rate of a three-dimensional cam surface can be appropriately adjusted by setting the sintered density for the net-shape-sintering process. The term xe2x80x9chole ratexe2x80x9d as used herein is the proportion of the total hole area to the surface area of the cam profile surface expressed in percentage.
The presence of holes contributes to an improvement in lubricant retention because a lubricant, such as an oil or the like, enters the holes. Therefore, an increase in the hole rate further reduces the friction on the cam profile surface, that is, further improves the friction characteristic of the cam, so that slide abrasion can be more effectively curbed.
An increase in the hole rate also increases the roughness of the cam profile surface, so that the resistance to pitting decreases. However, the present inventors have ascertained that if the hole rate is within the range of 5 to 10%, a sufficient pitting resistance can be attained while the friction is curbed within a permissible magnitude (see FIG. 2). Therefore, according to the first aspect, it is possible to achieve a further improved durability in the three-dimensional cam having a cam profile that changes along the rotating axis, while securing a high productivity.
In accordance with a second aspect of the invention, in a method for producing a three-dimensional cam having a cam profile shape that changes along a rotating axis by net-shape-sintering, a sintered density for the net-shape-sintering is set to about 7 to 7.4 grams per cubic centimeter.
As described above, net-shape-sintering is able to produce a three-dimensional cam with a high productivity while securing a sufficiently high precision in forming the three-dimensional cam. Furthermore, since a surface of the three-dimensional cam produced by the net-shape-sintering has holes at a hole rate of 5 to 10%, a high durability is secured.
In the production method of the second aspect of the invention, a sintering material for the net-shape-sintering is compacted so that the density of the sintering material, that is, the sintered density, becomes 7 to 7.4 grams per cubic centimeter. According to this production method, a frame mold having a molding surface for molding a shape of the three-dimensional cam is filled with a material powder of the three-dimensional cam. The material powder is press-molded by the frame mold into the shape of the three-dimensional cam at a density of about 7 to 7.4 grams per cubic centimeter. The molded body is sintered at a predetermined temperature. If the three-dimensional cam is produced by the above-described production method, the hole rate of the surface of the three-dimensional cam can be set to 5 to 10%.
If the net-shape-sintering is performed at a sintered density as mentioned above, the hole rate of a surface of the three-dimensional cam can be set to 5 to 10%.
Therefore, according to the second aspect, it is possible to produce a high-durability three-dimensional cam while securing a sufficiently high productivity.
In the three-dimensional cam production method of the second aspect of the invention, when the three-dimensional cam is removed from the frame mold during the net-shape-sintering, the mold withdrawal direction may be set to such a direction that the frame mold and the cam profile surface do not slidingly contact each other.
If the cam profile surface of a three-dimensional cam rubs against a frame mold when the three-dimensional cam is removed from the frame mold during production of the three-dimensional cam by net-shape-sintering, external edge portions of holes on the cam profile surface may deform so that holes formed by the sintering may be crushed. As a result, the hole rate decreases, so that a desired hole rate may not be achieved.
Therefore, if the three-dimensional cam is removed from the frame mold by withdrawing the frame mold in such a direction that the frame mold and the cam profile surface do not slidingly contact each other, a desired hole rate can always be achieved and, therefore, a high-durability three-dimensional cam can be produced with an even higher quality.